AD7866 datasheet(13/20 Pages) AD

13 / 20 page

REV. 0

AD7866

–13–

Digital Inputs

The digital inputs applied to the AD7866 are not limited by the

maximum ratings which limit the analog inputs. Instead, the

digital inputs applied can go to 7 V and are not restricted by the

VDD + 0.3 V limit as on the analog inputs. See maximum ratings.

Another advantage of SCLK, RANGE, REF SELECT, A0,

and

CS not being restricted by the V

DD + 0.3 V limit is the fact

that power supply sequencing issues are avoided. If one of

these digital inputs is applied before VDD, there is no risk of

latch-up as there would be on the analog inputs if a signal greater

than 0.3 V were applied prior to VDD.

VDRIVE

The AD7866 also has the VDRIVE feature. VDRIVE controls the

voltage at which the serial interface operates. VDRIVE allows the

ADC to easily interface to both 3 V and 5 V processors. For

example, if the AD7866 was operated with a VDD of 5 V, the

VDRIVE pin could be powered from a 3 V supply, allowing a large

dynamic range with low voltage digital processors. For example,

the AD7866 could be used with the 2

× VREF input range, with a

VDD of 5 V while still being able to interface to 3 V digital parts.

REFERENCE SECTION

The AD7866 has various reference configuration options. The

REF SELECT pin allows the choice of using an internal 2.5 V

reference or applying an external reference, or even an individual

external reference for each on-chip ADC if desired. If the REF

SELECT pin is tied to AGND then the on-chip 2.5 V reference

is used as the reference source for both ADC A and ADC B. In

addition, pins VREF, DCAPA, and DCAPB must be tied to decoupling

capacitors (100 nF, 470 nF, and 470 nF recommended, respec-

tively). If the REF SELECT pin is tied to a logic high, then an

external reference can be supplied to the AD7866 through the

VREF pin to overdrive the on-chip reference, in which case decoupling

capacitors are required on DCAPA and DCAPB again. However, if

the VREF pin is tied to AGND while REF SELECT is tied to a

logic low, then an individual external reference can be applied to

both ADC A and ADC B through pins DCAPA and DCAPB,

respectively. Table II summarizes these reference options.

For specified performance the last configuration was used, with

the same reference voltage applied to both DCAPA and DCAPB.

The connections for the relevant reference pins are shown in the

typical connection diagrams. If the internal reference is being

used, the VREF pin should have a 100 nF capacitor connected to

AGND very close to the VREF pin. These connections are shown

in Figure 12.

DCAPB

DCAPA

VREF

470nF

AD7866

470nF

100nF

Figure 12. Relevant Connections When Using

Internal Reference

Figure 13 shows the connections required when an external

reference is applied to DCAPA and DCAPB. In this example the same

reference voltage is applied at each pin; however, a different volt-

age may be applied at each of these pins for each on-chip ADC.

An external reference applied at these pins may have a range from

2 V to 3 V but for specified performance it must be within

±1%

of 2.5 V. Figure 14 shows the third option which is to overdrive the

internal reference through the VREF pin. This is possible due to the

series resistance from the VREF pin to the internal reference. This

external reference can have a range from 2 V to 3 V, but again to get

as close as possible to the specified performance a 2.5 V reference is

desirable. DCAPA and DCAPB decouple each on-chip reference buffer

as shown in Figure 15. If the on-chip 2.5 V reference is being used,

and is to be applied externally to the rest of the system, it may

DCAPB

DCAPA

VREF

AD7866

VREF

REF SELECT

Figure 13. Relevant Connections When Applying an

External Reference at DCAP A and/or DCAP B

DCAPB

DCAPA

VREF

470nF

AD7866

470nF

VREF

REF SELECT

VDRIVE

Figure 14. Relevant Connections When Applying an

External Reference at VREF

Table II. Reference Selection

Reference Option

REF SELECT

VREF

DCAP A and DCAPB

Internal

Low

Decoupling Capacitor

Externally through VREF

High

External Reference

Decoupling Capacitor

Externally through

DCAP A and/or DCAPB

Low

AGND

External Reference A and/or

Reference B

NOTES

1Recommended value of decoupling capacitor = 100 nF.

2Recommended value of decoupling capacitor = 470 nF.

AD7866 датащи(PDF) 13 Page - Analog Devices

AD7866 датащи(HTML) 13 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

датащи скачать

ссылки URL